The long term objective of this project is to improve the therapeutic index of radiation therapy for the treatment of prostate cancer (PCa). Radiation therapy is one of two primary treatments for clinically-localized PCa and is the principal therapy for locally-advanced disease associated with a higher grade, stage and/or PSA. While the success rate for both radiation and surgery is high for low-grade organ-confined disease, the estimated ten year disease-free-survival for advanced disease is less than 50%. Therefore, a means to improve the treatment of patients with clinically-localized high stage and/or grade prostate cancer would significantly decrease the morbidity and mortality of this disease. To address this we developed prostate-targeted RNA interference (RNAi) agents that selectively inhibit DNA repair pathways in prostate cells. Tissue-specific targeting was achieved through an RNA aptamer, A10-3, which binds to the Prostate Specific Membrane Antigen (PSMA) on the cellular surface and is then internalized into cells. The conjugated short hairpin RNAs (shRNAs) are then processed by cellular RNAi machinery, leading to knockdown of specific DNA repair mRNA and proteins. In the previous funding cycle we demonstrated that these agents, when directly injected into the tumor, significantly enhance the therapeutic index of external beam radiation therapy. Here we propose to continue the development of this strategy by generating chemically synthesized aptamer-siRNA radiation sensitizing agents and evaluating their safety as intraprostatically injected agents in a Phase I clinical trial. In addition to this translational aim, we will perform pre-clinical studies to determine whether the aptamer-siRNA radiation sensitizing agents are effective following intravenous injection. If successful, this mode of administration would provide a secondary route for radio-sensitizing locally advanced PCa, it would provide a novel means to improve the efficacy of external beam radiation therapy for the management of bone pain, and finally it would provide a means to enhance the efficacy of systemically targeted radiotherapeutics.